@apollo/client

Minor Changes

• Implement preview support for the @defer directive.
  @alessbell and @benjamn in #10018

• Implement useFragment_experimental hook, which represents a lightweight live binding into the ApolloCache, and never triggers network requests of its own.
  @benjamn in #8782

• Allow registering named fragments with InMemoryCache to support using ...NamedFragment in queries without redeclaring NamedFragment repeatedly in every query that uses it.
  @benjamn in #9764

• Support onError callback for useSubscription hook.
  @jeroenvisser101 in #9495

• Implement preserveHeaderCase option for http context object, enabling preserved header capitalization for non-http-spec-compliant servers.
  @mrdoombringer in #9891

Patch Changes

• Delay calling onCompleted and onError callbacks passed to useQuery using Promise.resolve().then(() => ...) to fix issue #9794.
  @dylanwulf in #9823

• Replace concast.cleanup method with simpler concast.beforeNext API, which promises to call the given callback function just before the next result/error is delivered. In addition, concast.removeObserver no longer takes a quietly?: boolean parameter, since that parameter was partly responsible for cleanup callbacks sometimes not getting called.
  @benjamn in #9718

• Allow preserving header name capitalization when creating an HttpLink with createHttpLink({ uri, preserveHeaderCase: true }). Otherwise, header names are converted to lowercase to prevent case-sensitivity bugs.
  @MrDoomBringer in #9891

• Make queries with a pollInterval respect the no-cache fetch policy, instead of writing polled results into the cache.
  @MrDoomBringer in #10020

• deprecated

  Deprecate the onSubscriptionData callback in favor of a new onData callback for the useSubscription hook. Deprecate the onSubscriptionComplete callback in favor of a new onComplete callback for the useSubscription hook.
  @jerelmiller in #10134

Potentially disruptive

• The optional subscribeAndCount testing utility exported from @apollo/client/testing/core now takes a single generic TResult type parameter, instead of TData. This type will typically be inferred from the observable argument type, but if you have any explicit calls to subscribeAndCount<TData>(...) in your own codebase, you may need to adjust those calls accordingly.
  @benjamn in #9718